Silver eel mortality during downstream migration in the Meuse: comparing telemetry study 2010-2012 to 2002-2006

During the descend to sea, silver eel face different hazards causing migration delay or mortality (e.g. dams, hydropower stations (HPS), weirs, ship locks, fisheries, predation). In 2010 fisheries was closed from September – November 30th and from April 1th 2011 the commercial eel fisheries in the river Meuse was closed for a undetermined period, due to dioxin pollution. To compare general mortality percentages with earlier studies (2002 and 2004), additional telemetry studies were carried out in 2010 and 2011. Compared to earlier studies more detection stations were used to estimate the mortality of the migration silver eel. This report attempts to answer two research questions: 1) Did the closure of the fisheries starting in 2010 result in a lower mortality rate for downstream migrating silver eel in the Meuse compared to previous studies in 2002-2006? 2)What is the delayed and direct Hydro power station (HPS) mortality for migrating silver eel in the Meuse during 2010-2012? In 2010 a batch of 150 female silver eel with surgically implanted transponders was released in the river Meuse and they could be followed with an array of detection stations of the Nedap TrailTM System. From the detection data mortality estimations were obtained. The present report is restricted to analysing migration routes and mortality estimations for the batch released in 2010. In accordance with the methodology used in 2002 (16 – 26% mortality) and 2004 (25 – 34% mortality) there was no big difference found in mortality estimations in 2010 (13 – 26% mortality). A comparison between the methodology used in previous studies (13 – 26% mortality) and new mortality estimations using more detection stations (22-25%) suggests that a more precise mortality estimation must be found in the upper limits of the mortality ranges derived from previous studies. Based on telemetry we found a minimum percentage of delayed and direct mortality of at least 20% of the migrating batch (121 eel). This percentage was corrected for other factors causing mortality (e.g. natural mortality, disease, predation etc.)